A power converter in the context of the present disclosure typically comprises an inverter. An inverter may comprise semiconductor switches configured into an inverter bridge. The inverter generates output voltages or currents by modulating the semiconductor switches into a conducting or non-conducting state.
A power converter may be coupled with an output filter in order to improve the quality of the power produced by the power converter. The filter may be an LC or an LCL filter, for example. In some applications, the LCL filter may have a direct connection from the star point of capacitors of the filter to a DC bus of the power converter. In the present disclosure, such a configuration is called a power converter with virtual ground (VG) connection. The virtual ground connection may be used for limiting high frequency fluctuation of the inverter DC bus with respect to a ground potential.
Part of the output filter may form a common-mode resonance (CMR) circuit that carries a circulating resonance current on the virtual ground connection. For example, if a power converter comprises a high-performance (i.e. low-loss) LCL filter with a virtual ground connection, a strong resonance may arise via a common-mode LC circuit as the natural damping of the circuit (i.e. losses at a resonance frequency) may be very low due to the use of high-performance, low-loss materials. Even when a common-mode voltage control reference of the inverter does not excite the common-mode resonance, a wide dispersing spectrum of the common-mode voltage (due to inverter modulation) may give rise to this resonance at almost every operating point of the inverter.
Mitigating the common-mode voltage effects at the resonance frequency may be desirable since a common-mode resonance may cause additional losses and performance degradation of the inverter (e.g. degradation in efficiency, EMC and grid current control performance). For example, adverse effects of the CMR may be present in photovoltaic (PV) inverters where the CMR may cause increased leakage currents via PV modules' stray capacitances. The leakage current may have certain limit values that are dictated by standards and, therefore, minimizing any additional high-frequency content on the inverter DC bus may be desirable.
Adverse effects of the resonant component of the common-mode voltage may be reduced by using passive or active damping methods, for example. Passive common-mode damping may be implemented with auxiliary circuitry such as resistive components mitigating the common-mode component, for example. However, such additional components may increase the cost of the system and reduce its performance. Active common-mode damping methods may also require the use of additional sensors and/or may not operate as intended under non-ideal operating conditions.